In a DWDM (dense wavelength division multiplexing) optical communications system, 40, 80, or 128 number of channels separated from each other by a wavelength difference of 0.4 nm or 0.8 nm are modulated at several tens of Gbs and then transmitted simultaneously through a single optical fiber, thereby enabling the expansion of a data transmission rate up to several terabytes per second. Since, however, different wavelengths suffer different absorption losses within the optical fiber and, further, optical characteristics of devices employed in the optical communications system, e.g., an optical source, an optical amplifier, a photo detector, a wavelength division multiplexer and demultiplexer, etc., are varied depending on a wavelength involved, there occur differences in optical intensity between the channels. Thus, an optical attenuator has been conventionally employed in order to adjust the differences in optical intensity between the channels.
Conventionally, there exist three types of optical attenuators.
First, there is an optical attenuator which changes continuously a path of light coming from an input fiber in order to adjust an amount of light to be introduced into output fiber. In an optical attenuator of this type, a reflector perpendicular to optic axes of the optical fibers is partially inserted between two optical fibers aligned in a straight line to reflect some portion of the light. In addition to the above-described configuration, it is also possible that two optical fibers are aligned parallel to each other in a direction perpendicular to a reflector. In this configuration, the amount of light that comes from an input fiber and then is reflected to output fiber can be controlled by varying an angle of the reflector. Besides, the distance between two optical fibers can be varied to adjust the amount of diffracted light introduced to an opposite optical fiber. Furthermore, optic axes of two optical fibers can be separated in a direction perpendicular to a propagation direction of light.
However, since this type of optical attenuator using the above-described various attenuation methods does not have wavelength selectivity, it involves steps for separating channels from each other and adjusting differences in optical intensity of the channels. Accordingly, the system structure has been complicated.
Second, there exists an optical attenuator using transmission characteristics of a Fabry-Perot cavity. Specifically, employed in this optical attenuator is the fact that a transmittance for a certain wavelength is differed as a distance between two reflectors is changed. If a reflectance of the reflectors is high, a transmittance of a Fabry-Perot cavity for a certain channel near a resonant wavelength gets rapidly changed as a function of the distance between reflectors though a channel selectivity is increased. Accordingly, it is required to regulate the distance between the reflectors very minutely, which proves to be difficult and troublesome. If the reflectance of the reflectors is low, on the other hand, the gradient of transmittance relative to the distance between reflectors is lowered, so that a transmitted power can be easily adjusted. However, the channel selectivity is decreased that light of neighboring channels can be transmitted concurrently. Thus, it is required to separate the channels from each other as in the case of the first type of optical attenuator.
Thirdly, there is an optical attenuator using a Faraday effect. In this type of attenuator, light is polarized by a polarizer at an input terminal, and then the polarization direction changes while the light passes through a material having a Faraday effect. A transmitted power is controlled by adjusting an polarization angle of the light relative to a polarizer located at an output terminal. In this method, however, light having only one specific polarization direction can be transmitted through the polarizer at the input terminal, so that loss rate of randomly polarized light reaches 50% or more. Further, in case polarized light emitted from a laser is employed, there exists a necessity for arranging the polarizer at the input terminal parallel to a polarization state of the laser. Furthermore, since it is very difficult to arrange all of the to-be-reflected light to be perpendicular to the to-be-transmitted light, the third type of optical attenuator also requires a process for separating the channels from each other by employing an additional device, as in the above-cited two cases.